1. Field of the Invention
The present invention generally relates to a tensed shadow mask assembly utilized in a cathode ray tube.
2. Description of the Prior Art
It is well known that a cathode ray tube utilized as a display of, for example, a television receiver set employs a shadow mask assembly which is made of a perforated thin metallic plate or foil. In the case of a color cathode ray tube, the perforated thin metallic plate or foil has a multiplicity of triads of minute circular apertures defined therein in a pattern corresponding to the triads of phosphor dots on the inner surface of the faceplate, each of the triad corresponding to the number of the primary colors.
When it comes to the manner by which the shadow mask assembly is supported inside the envelope in the vicinity of the luminescent phosphor-deposited screen, two support systems are generally utilized; one of them comprises securing the shadow mask at its peripheral edge portion to the funnel section of the cathode ray tube through a rigid frame member while the shadow mask has been formed to have a generally convex shape, and the other of them comprises securing the shadow mask, flat in shape, at its peripheral edge portion under tension to a rigid frame member which is in turn secured to the funnel section of the cathode ray tube. The shadow mask assembly utilized in connection with the generally flat phosphor-deposited screen is referred to as "flat-tensed shadow mask assembly" or, simply, "tensed shadow mask assembly".
In any event, the difference between the shadow mask support systems is discussed in U.S. Pat. No. 2,690,518, issued Sept. 28, 1954, to N. F. Fyler et al., and the details of the flat-tensed shadow mask assembly are disclosed in numerous patent publications including, for example, U.S. Pat. No. 2,755,402, issued Jul. 17, 1956, to A. Morrell.
Which one of these two systems is to be employed for the support of a particular shadow mask assembly depends on the shape of the phosphor-deposited screen of the cathode ray tube. Specifically, where the phosphor-deposited screen as a whole is generally spherical having a curvature corresponding to that of a portion of the sphere, the use of the shadow mask assembly having the generally convex shape is recommended. On the other hand, where the phosphor-deposited screen as a whole is generally flat, and particularly where the shape of the phosphor-deposited screen is such that the product of the maximum outer diameter of the phosphor-deposited screen multiplied by the average curvature of the same is of a value not greater than 0.3, the use of the flat-tensed shadow mask assembly is recommended.
In any event, the recent trend is that the flatness of the phosphor-deposited screen has come to be considered one of the factors that affect the quality of pictures displayed on the screen of the cathode ray tube. To cope with this recent trend, improvement in the flat-tensed shadow mask assembly has come to be one concern of important studies in the art.
An example of conventional flat-tensed shadow mask assemblies utilizable in association with the generally rectangular screen of the cathode ray tube is illustrated in FIG. 4. The flat-tensed shadow mask assembly generally identified by 1 comprises a generally rectangular perforated thin metallic plate 2 having a pattern of minute apertures 3 defined regularly for the passage of electron beams therethrough, and a correspondingly rectangular frame member generally identified by 10 used to support the perforated plate 2 while the latter is held under tension. The frame member 10 is made of metal so rigid as to permit the shape of the frame member 10 to withstand against the relatively high tension developed in the perforated plate 2 when the latter is secured thereto.
The frame member 10 is four-sided in shape opening at a central area thereof, and is comprised of a generally rectangular frame 11 and a flange 12 of predetermined width protruding laterally outwardly from the rectangular frame 11. The perforated plate 2 is, while having been tensed in all directions, secured at its peripheral edge portion to a flange face 13 of the flange 12 by means of a row of spot-weld desposits shown by the phantom line 20. Since the joint between the perforated plate 2 and the frame member 10 is required to have a sufficient rigidity, a reinforcement plate (not shown) similar in shape to the contour of the flange 12 may be subsequently welded to the flange 12 with the peripheral edge portion of the perforated plate 2 sandwiched therebetween. Alternatively, the reinforcement plate, the peripheral edge portion of the perforated plate 2 and the flange 12 may be welded together at the time of fitting of the perforated plate 2 to the frame member 10.
The shadow mask assembly 1 including the perforated plate 2 and the frame member 10 so connected together as hereinabove described is thereafter placed inside the funnel section of the envelope adjacent the phosphor-deposited screen of the cathode ray tube and retained in position by means of a suitable retaining mechanism (not shown) including, for example, tension springs connected to the frame member 10.
According to the prior art, the frame member 10 used to support the perforated plate 2 to complete the flat-tensed shadow mask assembly 1 has a substantial weight and is expensive to make. This is because the perforated plate 2 is highly tensed and, therefore, the frame member 10 must have a sufficient physical strength enough to withstand against any possible deformation which would occur under the influence of the tension imparted to the perforated plate 2. Of numerous deformations which the perforated plate 2 may suffer from during the use of the cathode ray tube, a warp is one of the major factors that affect the quality of picture reproduction and are therefore somewhat intensively studied.
The development of the warp in the flat-tensed rectangular shadow mask assembly will now be discussed with particular reference to FIGS. 5 to 7 in which, while the actual shape of the perforated plate 2 is shown by the single-dotted line, the perforated plate 2 is, for the sake of simplicity, shown as flat and having four right-angled corners represented by respective points A, B, C and D, with the center thereof shown by a point O. In an ideal configuration, the perforated plate 2 is completely flat with all five points A, B, C, D and O lying in the same plane, and no substantial moment tending to induce the warp occurs in the shadow mask assembly 1 including the frame member 10 even though the perforated plate 2, when held taut, may exhibit a tendency to resist the tension imparted thereto.
However, when an external force or impact is applied to the shadow mask assembly causing the perforated plate 2, then tensed in all directions, to deform in such a way as to have the points A and C displaced a distance towards points A1 and C1 in a direction perpendicular to the plane of the perforated plate 2 as shown in FIG. 6 and, consequently, the line drawn through the points A, O and C, which ought to remain straight, bends as shown by the solid line in FIG. 6. A force, acting to shorten the distance between the points A1 and C1, develops in the perforated plate 2. consequent upon this, a moment develops in the shadow mask assembly 1 itself, causing the latter to deform. This moment is substantially proportional to the force necessitated to displace the point A or C to the point A1 or C1, respectively, and, therefore, the moment increases, once the corner-to-corner bending as shown in FIG. 6 takes place, to further increase the corner-to-corner bending.
At the same time, the tension acting between the points A1 and C1 to draw these points A1 and C1 close towards each other is accompanied by the development of a pulling force by which the center point O tends to displace towards a point O1 in a direction perpendicular to the plane of the perforated plate 2. Once the center point O is consequently displaced even the slightest distance towards the point O1, another moment tending to deform the shadow mask assembly 1 as a whole develops on the line drawn through the points B, O1 and D, resulting in the displacement of the points B and D to points B1 and D1 as shown in FIG. 7. The direction in which the moment tending to bring about the corner-to-corner bending of the line drawn through the points B, O and D acts is counter to the direction in which the moment that has brought about the bending of the line drawn through the points A, O and C has acted on the perforated plate 2, and, therefore, the shadow mask assembly 1 as a whole is deformed in a manner as shown in FIG. 7.
The perforated plate 2 once so deformed will no longer deform when the force tending to deform the perforated plate 2, as discussed above, is brought in equilibrium with the drag force developed in the frame member 10 as a result of the deformation of the perforated plate 2. In any event, the deformation which may occur in the shadow mask assembly 1 is such that, since the frame member 10 is rectangular in shape and has a substantial rigidity, displacement of the portions along a pair of diagonal lines tends to be more considerable than the displacement of other portions of the perforated plate 2.
In view of the foregoing, the frame member 10 used in the conventional shadow mask assembly must be robust enough to withstand against the relatively high bending moment and must, therefore, be manufactured having a substantial weight and will be expensive, as hereinbefore discussed.
U.S. Pat. No. 3,109,117, issued Oct. 29, 1963, to S. H. Kaplan discloses, in FIG. 8 thereof, the use of a circular perforated plate scalloped with respect to the center thereof so as to have its peripheral edge undulated at three points with respect to the center thereof. However, the purpose of the use of the scalloped feature in the perforated plate disclosed in this U.S. patent is to avoid the mislanding of electron beams traveling from the three-beam electron gun assembly towards the phosphor dots on the screen. More specifically, it avoids an `azimuth error` or a distortion of both the phosphor and the beam triads pronounced at the outer periphery of the scan raster of the cathode ray tube.